1. Field of the Invention
This invention is related in general to the process of manufacture of cationic polymer thin films by vacuum vapor deposition. In particular, it pertains to a process of flash evaporation, vapor deposition, and radiation curing of cation-polymerizable monomers and oligomers.
2. Description of the Related Art
Inorganic and polymeric coatings are used on various substrates to add or promote desirable properties for particular applications. For example, foils used to preserve food need to have very low permeability to oxygen. Therefore, it is desirable and sometimes necessary to modify the physical properties of polymeric films to improve their suitability for the intended purpose. Preferably, the films are directly formed with a composition and molecular structure characterized by the desired properties.
Thin films of metals, ceramic and polymers are created by deposition onto appropriate substrates by a variety of known processes, most notably through film formation by wet chemistry or vapor deposition. Chemical processes produce soluble thermoplastic as well as insoluble thermoset polymers and involve the use of solvents; thus, thin film formation is achieved through solvent diffusion and evaporation. As a result, these processes require relatively long residence times and the undesirable step of handling solvents.
Vacuum deposition processes involve the flash evaporation of a liquid monomer in a vacuum chamber, its deposition at room temperature or on a cold substrate (referred to in the art as xe2x80x9ccrycondensationxe2x80x9d), and the subsequent polymerization by exposure to a high-energy source of radiation, such as electron beam or ultraviolet radiation. As illustrated schematically in FIG. 1, the liquid monomer from a supply reservoir 12 is fed through a capillary tube 14 and an atomizer 16 into the heated evaporator section of a vacuum deposition chamber 10, where it flash vaporizes under vacuum. The resulting monomer vapor is then passed into a condensation section of the unit where it condenses and forms a thin liquid film upon contact with the cold surface of an appropriate substrate, such as a film 18, which in turn is in contact with a cold rotating drum 20 as it progresses from a feed roll 22 to a take-up roll 24. A metal vaporization unit 26 may also be used to deposit in line a thin metal layer on the film 18 for multilayer deposition. The liquid deposited film is then cured by exposure to an electron-beam or ultraviolet radiation source 28. A duplicate polymer coating system with a corresponding liquid monomer supply reservoir 12xe2x80x2, capillary tube 14xe2x80x2, atomizer 16xe2x80x2, and radiation source 28xe2x80x2 may be utilized to apply multiple monomer coats over the film substrate 18. Since the ultimate objective is the formation of solid films, the initial liquid monomer must be capable of polymerization and contain enough reactive groups to ensure that a sufficiently large polymeric molecule results and yields a solid product. A conventional plasma-gas treating system 30 is also used to clean and prepare the film 18, if desired.
This conventional approach of utilizing a polymerizable monomer as the raw material for thin-film forming processes has been followed over the years because it is not possible to vaporize the final polymeric product under the range of operating conditions of a commercially viable vapor deposition chamber (typically, 10xe2x88x923 to 10xe2x88x926 torr and 70xc2x0 C.-350xc2x0 C.). The higher temperatures required to effect the vaporization of polymers having molecular weight greater than about 5,000 would destroy the polymer. Thus, the practice in the industry has been to identify or develop polymers having specific characteristics deemed advantageous for a particular film application. A solid thin film of the polymer is then formed on a target substrate by evaporating the corresponding monomer or oligomer, cryocondensing it as a monomer or olygomer in liquid form and polymerizing or cross-linking it to reach the required molecular weight to ensure its solidification. Many variations of this basic approach have been developed for particular applications, but conventional prior-art vacuum deposition processes involve the formation of a solid film by polymerization of a liquid monomer evaporated under vacuum or atmospheric conditions and recondensed on a cold surface to obtain the desired film characteristics.
The high rate of deposition and the better quality of the coatings produced make the vacuum film-forming process a commercially preferred technique. Therefore, considerable research has been conducted to develop processes for improving the properties of thin films obtained by polymerization of vacuum deposited monomers and oligomers. See, for example, U.S. Pat. Nos. 5,681,615, 5,440,446, 5,725,909, 5,902,641 and 6,010,751. A new approach to overcome some process limitations, involving the flash evaporation of oligomers, has been disclosed in U.S. Pat. No. 6,270,841, hereby incorporated by reference.
In the prior art, vacuum deposition and radiation curing have been used only with monomers and oligomers that polymerize via the free-radical polymerization mechanism. As such, mostly acrylates and methacrylates have been utilized to produce a variety of useful film products. When electron-beam radiation is applied, no initiator is needed because electrons are capable of creating the free radicals that initiate the polymerization. If ultraviolet or other high-energy photoradiation is used, free-radical photoinitiators such as aromatic ketone derivatives are used. These are non-ionic, easy-to-evaporate organic molecules.
By contrast, vacuum deposition of cation-polymerizable monomers or oligomers has not been available because conventional Lewis-acids and Bronstead-acids cationic initiators cause the polymerization reaction to start at room temperature before flash evaporation can be carried out. Even photoactive aryldiazonium cationic initiator salts are not sufficiently thermally stable to survive the flash evaporation process. In addition, earlier generations of vacuum equipment (atomizers, evaporators and nozzles) were not efficient for flash evaporation of mostly heterogeneous cationic systems that contain thermally stable, low-vapor-pressure cationic photoinitiator salts. Moreover, the commercial availability of cationic polymerization systems (monomers, oligomers and initiators) was limited.
These limitations thus made it impractical to attempt to use flash-evaporation vacuum-deposition technology for cationic polymerization systems, especially for commercial applications that require high rates of coating, superior properties, and low costs. As a result, the advantages afforded by the combination of flash-evaporation vacuum-deposition coating techniques (solventless, defect-free, ultra-thin, and in-line metallization) and cationically polymerized coatings (excellent adhesion, high barrier, electrochemical stability, high dielectric strength and low infrared absorption) have not been obtained in a single product.
Because of the poor solubility of cationic-photoinitiator salts in monomer or oligomer blends, most cationic polimerization reactions require solvents to produce the formation of homogeneous polymeric products. Solvent-based formulations are inherently incompatible with vacuum-deposition techniques. However, the process of flash evaporation followed by vacuum deposition overcomes the problem of photoinitiator solubility and produces a clear homogeneous coating because of the very short time between deposition and curing, which does not allow phase separation, even starting with a heterogeneous monomer/initiator blend.
During the last few years, progress has been made in different areas which set the stage for attempting the processing of cationically polymerizable monomers and oligomers as commercial coating materials via flash evaporation, vacuum deposition and radiation crosslinking techniques. Thermally stable, photoactive, cationic initiators (e.g., diaryliodonium, triarylsulfonium and ferrocenium salts) have been developed and have become commercially available. These cationic initiators are completely inactive at ambient conditions. They are themselves activated by either electron beam or ultraviolet radiation. Therefore, the possibility of combining them with cationically polymerizable monomers and oligomers in a flash-evaporation process became more realistic.
Thermally stable, photoactive, cationic initiators have been used extensively for conventional cationic polymerization processes, but mostly in solvent systems and only at atmospheric conditions. See, for example, U.S. Pat. No. 6,020,508. In related research work, non-ionic, metal free, organic soluble cationic photoinitiators were also synthesized and investigated for atmospheric systems (Mikhael et al, Nacromolecules, 28, 5951, 1995). These initiators were not initially thought to be good candidates for flash evaporation because of their very low volatility.
Besides the recent developments that led to the commercial availability of thermally stable cationic photoinitiator salts, a larger variety of cationically polymerizable monomers and oligomers (e.g., vinyl ethers, cycloaliphatic epoxy, glycidyl derivatives, styrene derivatives, divinyl styrene, oxetanes, vinyl pyridine, vinyl carbazole, vinyl imidazole) have also become commercially available. In addition, a new generation of monomer-evaporating units has become available with larger surface areas, multiple injectors, and shorter path to the nozzle, which justified attempting the flash evaporation of cationic photoinitiator salts (which have a much lower vapor pressure than the free radical photoinitiators used for acrylates) together with cationically polymerizable monomers and oligomers, hoping to avoid the premature thermal polymerization or salt separation. The present invention consists of a viable process for the production of polymer coatings from cation-polymerizable monomers and oligomers through flash evaporation, vacuum deposition, and radiation curing.
The main objective of this invention is the flash evaporation, vacuum deposition and radiation curing of cation-polymerizable, non-acrylate, monomers and oligomers.
Another objective is a process for producing cation-polymerized coatings at a high production rate.
Another object is the production of cation-polymerized coatings in a solvent-free environment.
Yet another object is the production of cation-polymerized coatings in high-barrier pinhole-free films.
Still another objective is a process that can be implemented to coat large surface areas.
Another goal is a process that produces cation-polymerized coatings in highly uniform, defect-free, ultra-thin films (0.01-10 microns).
Specifically, a goal of the invention is a process for manufacturing thin films of cation-polymerized polymer composites with superior barrier properties, electrochemical stability, dielectric strength and infrared transmittance than exhibited by prior-art (acrylate-based) coatings.
Another specific goal is a process that produces hybrid films of cation-polymerized polymers and inorganic materials, such as metals and/or ceramics.
Another objective is a procedure that can be implemented utilizing prior-art flash-evaporation vapor-deposition technology.
A final objective is a procedure that can be implemented easily and economically according to the above stated criteria.
Therefore, according to these and other objectives, the present invention consists of selecting a thermally stable photoactive (chemically inactive at ambient conditions) cationic initiator suitable for polymerizing non-acrylate cationically-polymerizable monomers or oligomers of interest and capable of vaporization under vacuum and temperature conditions of available flash-evaporation chambers. Such photoinitiator is mixed with monomers and/or oligomers similarly suitable for vaporization under the same vacuum and temperature conditions; and the mixture is flash evaporated and cryocondensed in conventional manner as a film on a cold substrate. The resulting vacuum-deposited, homogeneous, highly uniform layer is then cured with a high-energy radiation source that causes the cationic photoinitiator to dissociate into acidic species that initiate the cationic crosslinking reaction of the monomer/oligomer compounds in its deposited film form. As a result of the homogeneous, solventless pinhole-free nature of the vacuum deposition process, the thin-film polymer product does not suffer from the disadvantages attendant to prior-art atmospheric processes for cationically-cured polymers. In addition, because of the versatility afforded by vacuum deposition, hybrid films of such polymers with inorganic materials are also easily manufactured in-line during the same process at higher rate than that of conventional atmospheric coating processes.
Various other purposes and advantages of the invention will become clear from its description in the specification that follows and from the novel features particularly pointed out in the appended claims. Therefore, to the accomplishment of the objectives described above, this invention consists of the features hereinafter illustrated in the drawings, fully described in the detailed description of the preferred embodiment and particularly pointed out in the claims. However, such drawings and description disclose but one of the various ways in which the invention may be practiced.